What constitutes 'self-plagiarism'?

Last month, Nature News blog reported on an emminent chemist who was investigated for 'self-plagiarism' or duplicate publication.  Apparently a number of paragraphs were almost identical in a number of papers he published.  The most recent paper, published in Journal of the American Chemical Association, has since been retracted with the statement: "This article was removed by the publisher due to possible copyright concerns. The Journal’s Editor is following established procedure to determine whether a violation of ACS Ethical Guidelines to Publication of Chemical Research has occurred."  

Read the Nature news item and comments from readers here.  What are your views on self-plagiarism?  Let us know in the comments below.

UPDATE: here is the latest on this from Retraction Watch.